The present invention relates to a method and a device for starting an internal combustion engine in a hybrid drive of a vehicle.
For several years vehicles with so-called hybrid drives have existed as energy saving and environmental friendly alternatives to conventional internal combustion engines. A hybrid drive is usually defined as the combination of a number of different drive principles or the combination of a number of different energy sources for the respective type of drive. Therefore, a hybrid drive generally exhibits two different energy converters and two different energy accumulators. With very few exceptions, the practical implementation of the energy converters involves an internal combustion engine and an electric motor, and the energy accumulators involve a combustible fuel and a battery.
In a vehicle with a hybrid drive, both the internal combustion engine and the electric motor can be operated in a respectively optimal efficiency range. Excess energy, for example when braking or during passive coasting, is used via a generator for charging the battery.
When accelerating, the internal combustion engine and the electric motor usually work together, so that, in comparison to a typical internal combustion engine, a smaller one can be used. Since an internal combustion engine can deliver a high torque especially in a higher speed range, the provided electric motor is more suitable especially when starting from standstill, because it can provide a maximum torque even at low speeds. Therefore, at certain driving dynamics, both the engine and the motor can be started and stopped in order to achieve a driving mode that exhibits optimal energy consumption at high efficiency.
Especially when starting from a standstill or, for example, when accelerating after slowly driving through a tight curve, the electric motor is more suitable for accelerating. When then accelerating to a moderate speed or as far as up to a high speed, the internal combustion engine exhibits good efficiency. Therefore, the internal combustion engine is started as a function of the respective driving situations and is coupled, for example, by a clutch to the electric motor provided in the vehicle drive train.
For example, when braking or coasting, the electric motor acts as a generator and charges the battery in a corresponding recuperation phase. Usually hybrid vehicles are provided with an automatic transmission that is controlled by a control unit for obtaining the best possible efficiency for the conversion of potential electric or chemical energy into kinetic energy.
When starting the internal combustion engine, the electric motor is used as the starter. After a brief movement of the internal combustion engine the ignition is started. Then both the electric motor and the internal combustion engine must be adjusted to a common speed. Frequently the noises generated in engaging the individual drive possibilities are perceived by the driver as irritating.
Therefore, an object of the present invention is to provide an improved method for starting an internal combustion engine in a vehicle hybrid drive.
The method for starting an internal combustion engine in a vehicle hybrid drive and/or the corresponding device in accordance with the present invention exhibits the advantage that a brief overshooting of the target speed for the internal combustion engine generates a characteristic noise that is often acquainted with known vehicles. The result is an enhanced driving comfort.
According to the invention, a corresponding speed overswing beyond the target speed, which is, for example, the current speed of the electric motor, is produced. By specifying the corresponding speed differential and the length of the overswing by a predefined time period, the acoustical effect in starting the internal combustion engine can be precisely set. The resulting corresponding sound characteristics may be chosen, for example, so that they remind the driver of the starting of a sports engine.
In a contemplated variation of the inventive method, the following method steps are carried out:                monitoring at least one start criterion for starting the internal combustion engine;        coupling the internal combustion engine to the electric motor;        igniting the internal combustion engine; and        adjusting the speed of the internal combustion engine in such a manner that the speed exceeds over a predefined period of time and by a predetermined speed differential the target speed for generating the speed overswinger.        
Start criteria for switching on and coupling the internal combustion engine to the drive train of a corresponding vehicle are, for example, the typical vehicle dynamics such as an acceleration driving mode at a moderate or high speed, long trips at a moderate or high speed, during which the internal combustion engine exhibits better efficiency than the electric motor, or, for example, a critical charge state of the battery for the electric motor. The internal combustion engine is coupled to the electric motor, or, for example, by a controllable mechanical clutch.
Preferably upon reaching a maximum speed, which is equal to the sum of the target speed and the speed differential, the speed of the internal combustion engine is adjusted back to the target speed, preferably the current speed of the electric motor.
In a currently preferred embodiment of the inventive method, the combustion engine and the electric motor are coupled together in a connecting phase by a (e.g., mechanical) clutching device. In so doing, the coupling strength for generating the speed overswing is temporarily reduced before reaching the target speed. Thus, the internal combustion engine can temporarily at a higher speed on the engine motor.
The speed differential ranges preferably from 200 to 500 revolutions per minute; and the preferred period to time ranges, for example, from 0.5 to 1 second. Preferably while starting the internal combustion engine, a gear change of a hybrid vehicle transmission is suppressed. In an especially preferred embodiment of the method, the target speed is exceeded as a function of the selector switch. Thus, it is possible for the driver to prevent in accordance with his needs a speed overswing when starting the internal combustion engine.
The device for connecting an internal combustion engine to an electric motor-driven hybrid vehicle drive includes:                torque converter for coupling the vehicle drive with a transmission of the hybrid vehicle,        a clutching device for coupling the electric motor with the internal combustion engine; and        an engine control unit, that carries out a corresponding method for starting an internal combustion engine in the vehicle hybrid drive.        
Furthermore, there is preferably a selector switch for suppressing the speed overswing.